I love Estime and I think he will be a good NFL running back. I know little about Corum except he played for michigan so I have a natural distaste for him.
The only thing I've seen of Corum is when I'd watch other games and his highlights would pop up. From those he looked like a very good RB who I'd like a lot more if he played for ND.
My impression before looking at any stats or combine numbers was that he's smaller and faster than Estime but not better.
Having said this, if I take a neutral stance on them and consider what I've gleaned from a lifetime of watching risers and fallers in NFL combine/draft and I look at those combine numbers of what they did at the same venue on the same running surface...it's so easy to see why a RB who is 16lbs lighter, 4 reps stronger and 0.18 faster in the 40, with equal 10 yard split would get drafted higher.
Then if you look into their playing stats comparison over the last two seasons:
Estime 366/2261YDS 6.1AVG 29TD
Corum 505/2708YDS 5.4AVG 45TD
Looking at this from a neutral standpoint, Estime has a better yards per carry but Corum has 16 more rushing TDs over last two seasons. I would consider the wear on the players in favor of Estime as Corum is 23 while Estime is 20 and Corum has 139 extra carries over the last two seasons.
So when just looking at the numbers, which doesn't always dictate how good a player will actually be in the NFL (but definitely does dictate 'in most cases' how much a team will spend on them ie draft placement for rookies) it's pretty hard to find areas where Estime looks like the better investment.